People have tried to harness wind energy for thousands of years. Lately, with fossil fuels running out, techniques for converting the energy in the wind to other forms of energy and particularly electrical energy have become more popular. It is known to use wind turbines to extract the energy from the wind. It is also known to use kites to extract the energy from the wind. Kites can fly at an altitude where the wind speeds are more reliable than the wind speed at the height of the hub of a wind turbine. Whereas a wind turbine may have a height of 80 meters, kites can be flown at a typical height of 400 meters to 700 meters or higher. Kites are also easier to maintain and repair since they can be returned to ground level when not used, since the equipment for generating electric power are not required to be mounted at a height and since the kite can be reeled in and transported to a location where repairs can be carried out.
A system for the extraction of energy from the wind using a kite typically includes a kite connected to a base unit using a tether. In one type of system, the tether is wound on a drum in a base unit and the rotation of the drum as the tether pulls away from the base unit is used to generate other forms of energy, for example electricity. In another type of system a propeller or a rotor is provided on the kite and as the wind passes the propeller or rotor, the propeller or rotor is used to generate electrical power which can then be transmitted to the base unit.
To efficiently generate power using a kite, it is desired to have the kite flying through the air. A static kite on the end of a rope can only produce lift relative to the actual wind speed. However, when the kite is allowed to move, lift is obtained based on the apparent wind that is created by the motion of the kite relative to the actual wind. A tethered kite will have a small region of airspace in which it can generate a high tension in the tether but one which is still within the limits of the system and the power generation equipment. The centre of this region will lie directly downwind of the base unit and at an angle of elevation from the base unit dependent on, for example, the design of the kite, the limitations of the power generation equipment and the wind speed. The region in which the appropriate high tension can be generated will hereinafter generally be referred to as “the centre of the wind”. If the kite moves away from the centre of the wind, in either azimuth or elevation, the amount of tension it can generate in the tether may be less than optimum for the system for extracting the energy from the wind. It is therefore desirable to keep the kite near the centre of the wind and also to control the kite to move at a high speed in the wind. It has been suggested that a suitable flight pattern is obtained when the kite is controlled to fly in a loop forming a figure-of-eight from the point of view of the base station. However, when following a figure-of-eight flight pattern, regular large steering actions are required, which consumes power and destabilises the kite. It has also been suggested that a substantially circular path, from the point of view of the base station, may allow efficient extraction of wind power. With the tether pulling out, the actual flight path would of course be helical. A circular or helical flight pattern has been found to provide a consistent tension on the tether and a more even power stroke during flight. However, the physical size of the kite means that the speeds of the wing tips are quite different in a circular or helical flight path, which makes the design of the kite difficult.
The invention aims to improve on the prior art.